import java.io.*;
import java.util.*;

public class Main {
    static ArrayList<Animal> animals = new ArrayList<>();
    static int[][] dp;
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int meat = 0;
        int grass = 0;
        int sumMeat = 0;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x == 0) {
                meat++;
            } else if (x == 1) {
                grass++;
            } else {
                if (meat > 0 || grass > 0) {
                    animals.add(new Animal(meat, grass));
                    sumMeat += meat;
                    meat = 0;
                    grass = 0;
                }
            }
        }
        if (meat > 0 || grass > 0) {
            animals.add(new Animal(meat, grass));
            sumMeat += meat;
        }
        dp = new int[animals.size()][sumMeat + 1];
        for (int[] arr : dp) {
            Arrays.fill(arr, -1);
        }
        int ans = dfw(animals.size() - 1, sumMeat);
        if (ans >= Integer.MAX_VALUE / 2) {
            System.out.println(-1);
        } else {
            System.out.println(ans / 2);
        }
    }
    
    static int dfw(int idx, int value)  {
        if (value < 0) {
            return Integer.MAX_VALUE / 2;
        }
        if (idx < 0) {
            if (value == 0) {
                return 0;
            } else {
                return Integer.MAX_VALUE / 2;
            }
        }
        if (dp[idx][value] < 0) {
            dp[idx][value] = Math.min(dfw(idx - 1, value) + animals.get(idx).meat, dfw(idx - 1, value - animals.get(idx).sum()) + animals.get(idx).grass);
        }
        return dp[idx][value];
    }
    
    static class Animal {
        int meat;
        int grass;
        
        public Animal(int meat, int grass) {
            this.meat = meat;
            this.grass = grass;
        }
        
        public int sum() {
            return meat + grass;
        }
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}